What does it mean to let our thoughts drift by like clouds?

Shifting our awareness from what our thoughts mean to how they fluctuate is an attentional exercise that develops liberating abilities over time. 

Observing the movement of clouds can provide a glimpse into how we can relate to mental activity more objectively, but it oversimplifies things when the analogy is taken too literally.

"There's a lot more going on in your brain and your body than you can ever be aware of. And yet, most people identify themselves with this little flash-lit area of consciousness."

~ Rae Armantrout 

Mindfulness practice leads to better decision making due to the skills it develops over time.  It’s about paying attention to ordinary experience differently.